This invention relates to hot-fill plastic or polyester containers, and more particularly to an improved sidewall construction for such containers.
In the past, most plastic or polyester containers were used to contain liquids that are initially dispensed at room temperature or chilled. However, in recent years, there has been a significant increase in the demand for polyester containers for packaging "hot fill" beverages. "Hot-fill" applications impose additional mechanical stresses on the container structure which cause the container to be less resistant to deformation when the container is being handled or if it is dropped. The thin sidewalls of conventional polyester containers deform or collapse at hot fill temperatures. Moreover, the rigidity of the container decreases immediately after the "hot-fill" liquid is introduced into the container, making the container more susceptible to failure due to mechanical stresses. As the hot-filled liquid cools, it shrinks in volume which has the effect of lowering the pressure or producing a "hot-fill" vacuum in the container. The container must be able to sustain such internal pressure changes while maintaining its configuration.
Various methods have been devised to counter thermal instabilities. One method broadly involves heat treating the polyester to induce molecular changes which will result in a container exhibiting thermal stability. Other methods involve forming the polyester structure into a structural configuration which can maintain stability during hot fill. Thus, the hot-fill containers being produced have a generally cylindrical main body which is provided with a plurality of elongated vertically oriented panels. These panels, which are commonly referred to as pressure or vacuum absorption panels, are designed to flex inwardly after the container has been filled with a hot liquid to accommodate the inevitable volume shrinkage of the liquid in the container as the liquid cools. However, the inward flexing of the panels caused by the hot fill vacuum creates high stress points at the top and bottom edges of the pressure panels, and especially at the upper and lower corners of the panels. These stress points weaken the portions of the sidewall near the edges of the panels, allowing the sidewall to collapse inwardly during handling of the container or when containers are stacked together. The cylindrical label mounting area must support the wrap-around label and must absorb a vacuum without losing its cylindrical label mounting shape.
These problems could be alleviated by increasing the thickness of the container wall. However, increasing the wall thickness results in an increase in weight for the container and in the material cost of the finished container, which results are not acceptable to the container industry. Accordingly, attempts to solve this problem have been directed to adding reinforcements to the container sidewall.
In U.S. Pat. No. 4,863,046, there is disclosed a hot-fill container which has a cylindrical main body portion which includes a plurality of vertically oriented pressure panels separated by vertically elongated land areas. The vertically elongated land areas between the pressure panels are reinforced by vertical ribs. Each of the pressure panels includes a plurality of transverse, radially recessed rib segments within the panel which ensure that the panel moves uniformly. The pressure panels extend from just below the upper label bumper to just above the lower label bumper, minimizing the area for securing the label to the container body. Label placement is critical because the areas above and below the panels for placement of the upper and lower edges of the label are relatively small. This imposes significant constraints on the manufacturing tolerances in applying the label to the container.
In another hot-fill container, which is disclosed in U S Pat. No. 4,805,788, the container sidewall includes a plurality of vacuum collapse panels each of which has longitudinally extending ribs disposed at the sides of the collapse panels. The ribs extend within the sides of the vacuum panels and terminate at the tops and bottoms of the vacuum panels, increasing the rigidity of the container.
Another consideration is that certain markets require hot-fill containers with a one to two liter capacity while being characterized by a high aspect ratio, that is, the ratio of the vertical height of the container to the diameter of the container being greater than 2.5 to 1. One approach to producing such containers involved elongating an existing smaller capacity hot fill container of the type having an outwardly projecting window area in the center of the vacuum panel. This required lengthening the vacuum panel. However, the larger window limited the area for the window to flex inwardly in compensating for vacuum created during hot filling of the container so that the panel tended to buckle at its center. Moreover, under side loading pressure, the container collapsed at the base of the vertical column or land area separating adjacent panels.
The principle mode of failure in such containers was non-recoverable buckling, due to weakness in the lower label section, under vacuum, during handling of the containers between the cooling tunnel and the labeler. Essentially, the vertical column between two adjacent vacuum absorption panels buckled at the lower end of the panels, producing a flat section. This buckling is only recoverable if the container is "shocked" by striking its base with an abrupt force to "pop" the container geometry back to its normal shape. Containers which buckle in this way cannot be labeled properly.
One known hot-fill container includes a plurality of vertically oriented vacuum panels separated by vertically elongated land areas, and each vacuum panel includes an outwardly projecting center portion which is adapted to flex inwardly under vacuum conditions. A small upset provided at the top and bottom edges of the vacuum panel enables the vacuum panel to resist taking a permanent set when the vacuum panel is pushed inwardly. However, this upset was not effective to prevent the vertical land areas on either side of a vacuum panel from taking a permanent set when the land area is deflected inwardly.